This invention relates generally to proportional counter-type radiation detectors and more particularly to a proportional counter tube configuration for improvements in counting of low-energy photons (&lt;100 keV) at high count rates. The invention is a result of a contract with the U.S. Department of Energy.
In the field of radiation detection, cylindrical gas proportional counter tubes are used extensively due to their simplicity and rugged design. Proportional counter tubes are filled with a selected, ionizable gas for the efficient detection of charged particles, neutrons, and low-energy (&lt;100 keV) X-rays. To minimize radiation attenuation to the structural medium, the counter-wall thickness and material are selected to be essentially transparent to the radiation or particles being detected. Additional advantages of proportional counters are: operation at room temperature, good energy resolution, excellent signal-to-noise ratios owing to gas multiplication, and large dimensions (several meters long).
In some applications, the count-rate capability of cylindrical proportional counters is limited by the diffusion of electrons drifting toward the anode from the point of an ionizing event within the detector gas volume. The size of the electron cloud owing to diffusion is proportional to the total drift time. The diffusion process causes the electrons from each detected event to arrive at the anode over an extended period of time, thus degrading the rise time of the avalanche pulse detected at the anode. This slow rise time limits the pass band of the signal processing circuitry and thereby the count-rate capability of the proportional counter. The problem is especially significant in large diameter proportional counters (&gt;50 mm), at high gas pressures (20 atm). These parameters are required for detecting low energy photons (&lt;100 keV) with adequate detection efficiency (&gt;50%) at low operating voltages (&lt;5 kv).
The detection of these low energy photons at high count rates (&gt;10.sup.5 counts/sec) cannot be achieved with adequate detection efficiency in a conventional (coaxial) proportional counter tube unless the gas pressure is increased to increase the detection efficiency. For the hich count-rate operation, high drift velocity must be obtained throughout the whole gas volume, requiring impractically large anode diameters and high bias voltages. For example, the electron drift velocity for a 97% Xe-3% CO.sub.2 gas mixture is approximately 2 cm/.mu.sec. for a field strength-to-pressure ratio (E/p)&gt;4 mv(cm Pa).sup.-1. With the conventional, coaxial anode-cathode configuration, it is difficult to maintain E/p&gt;4 mv(cm Pa).sup.-1 throughout the counter volume unless a large anode diameter is used. This in turn requires impractically high operating bias voltages and extreme smoothness of the anode surface. The bias voltage must be high to produce adequate gas multiplication. The anode surface must be smooth in order to prevent counter breakdown caused by corona discharges from sharp points on the anode surface.
The present invention mitigates these problems because the multianode cage acts as a large diameter, virtual anode. The value of E/p is maintained at &gt;4 mv(cm Pa).sup.-1 anywhere between the cathode cylinders and this cage, with reasonable bias voltage (&lt;5 kv). Adequate gas multiplication is obtained owing to the high value of E/p in the vicinity of the individual anode wires. High count-rate capability and good detection efficiency for approximately 100 keV photons are needed in biomedical applications (e.g., low-dose transmission and emission radiography).